Bone and joint replacement materials have been useful for treating a wide variety of musculoskeletal disorders. Replacement materials can, for example, be designed to restore both lost structure and function, particularly for load bearing applications. Bones in normal, healthy condition carry external joint and muscular loads by themselves. Following the insertion of orthopedic screws and/or implants, the natural bone in the treated region will share its load-carrying capacity with the implanted materials. Thus, the same load that had been originally born by the bone itself will now be carried by the ‘composite’ new structure. For load bearing screws and implants, clinically available devices are typically metallic.
The requirements for orthopedic metallic implants can be broadly categorized as (1) biocompatibility between the material and the surrounding environment with little or no adverse cytotoxicity and tissue reaction; and (2) the mechanical and physical properties necessary to achieve the desired biophysical function. Some desired properties are, for example, low modulus, high strength, good ductility, excellent corrosion resistance in the body fluid medium, high fatigue strength and good wear resistance. Titanium (Ti) and its alloys are widely used in orthopedic and dental implants because of favorable mechanical properties, corrosion resistance, and biocompatibility. However, Ti is a bioinert material having minimal interaction with the surrounding tissue. Accordingly, osseointegration with Ti implants, which requires a time-dependent kinetic modification of the surface of the implant, can take a long time.
Successful implantation challenges can also occur when the metallic implant material is significantly stiffer than the adjacent bone. Internal load bearing functionality naturally performed by the bone, can now be mainly supported by implanted screws or other structural implants. Such stress “shielding” of the natural bone can, in some instances, alter the normal stress stimuli for bone growth, and the reduction of bone stresses relative to the natural situation causes bone to adapt itself by reducing its mass in a process of resorption around the implant. This resorption/bone loss effect can cause micromotion of the screws/implants in response to external loads and could further damage the interfacing bone layer and anchorage performances subsequent to possible loosening of the screw/implant [1].
Infection is also a possible side effect often associated with implants and bone replacement surgeries. Infections, in some cases, may require removal of a surgically administered prosthesis or cause a significant delay in post-surgical healing. This is often due to the accumulation of microbial plaque or biofilm development on implants, screws or plates, which can contribute to recurrent infections as well as cause bone loss or prevent the necessary bone deposition for anchoring the surgical implant.